High performance superalloys, such as nickel- or cobalt-based superalloys, are being increasingly employed in various types of gas turbine engines used, for example, in the propulsion and power generation industries. High pressure components (e.g., airfoils) of high performance and high fuel efficiency turbine engines generally have a hollow core made of nickel- or cobalt-based superalloy and these components are subjected to corrosive exhaust gases at extremely high temperatures of up to 1150.degree. C. during the operation of a gas turbine engine. As a result, these components are prone to oxidation damage. Various solutions have been tried to prevent such oxidation damage.
One means typically employed is to provide the components with an environmentally resistant coating typically provided by an aluminum-rich alloy, such as aluminide, whose surface oxidizes to form an aluminum oxide (alumina) scale at elevated temperatures. Such a scale provides a tough, adherent layer that is highly resistant to oxidation and corrosion attack. However, one of the major problems faced by these environmentally resistant coatings is the spallation of the alumina layer thus formed. Upon subsequent cooling of the turbine blade, the thermal mismatch in the coefficients of thermal expansion of the underlying superalloy substrate and the alumina layer disposed thereon leads to an enormous compressive stress on the alumina layer. Thus, when the alumina layer is repeatedly subjected to such intense thermal cycles, spallation of the alumina layer frequently occurs. This in turn triggers regeneration of alumina when the underlying exposed aluminide layer is oxidized by the air at high temperatures produced during the normal working cycle of a gas turbine engine. This regeneration process depletes available aluminum from the underlying aluminide layer disposed on the surface of the substrate, ultimately limiting the life of the coated component. Furthermore, when a thermal barrier coating is applied on the alumina layer, any spallation of the underlying alumina layer results in loss of the overlying thermal barrier coating with grave consequences. Therefore, creating a spallation-resistant alumina layer is critical.
One example of such a spallation-resistant alumina interfacial layer is disclosed in the U.S. Pat. No. 4,880,614 where a high purity alumina interfacial layer is provided between the metallic substrate and the ceramic thermal barrier coating.